A. Field of the Invention
The present invention relates to methods and systems for providing two-way communication capability between a central network and a mobile unit over a relatively large area, and more particularly to such methods and systems which allow for rapid communication of large messages and efficient use of system resources.
B. Description of the Related Art
Conventional two-way portable/mobile wireless messaging systems often provide a variety of services to subscribers. Conventional messaging systems in particular provide one-way services using store and forward techniques to mobile receivers carried by the subscriber. A fundamental goal of two-way messaging systems is to provide a network of interconnected transmitters and receivers which provides sufficient transmitted signal strength and receive capability to uniformly cover a geographic region. Some conventional messaging systems provide the message to the user on a small viewing screen on the mobile unit.
However, such conventional systems often suffer from problems associated with low system throughput, evidenced by slow message delivery and message size limitations and do not provide an acknowledgment feature wherein the mobile unit transmits an acknowledgment signal to the system to acknowledge receipt of the message from the system. Generally, system throughput refers to the overall communication capability of a system as defined by the total amount of message data from the system to the mobile units transferred by the system during a given period of time divided by the frequency bandwidth necessary to transmit the message data and may be measured in bits transferred per Hz. Further, such conventional systems suffer from technical problems preventing consistent wide area coverage and would require extremely wide portions of valuable frequency bandwidth to achieve acceptable system throughput rates.
Simulcast technology in communication systems was originally developed to extend transmitter coverage beyond that which could be obtained from a single transmitter. Over time, however, simulcasting has evolved into a technique capable of providing continuous coverage to a large area.
Generally, simulcast technology provides multiple transmitters, operating on substantially the same frequencies and transmitting the same information positioned to cover extended areas. As shown in FIG. 1, transmitter 100 generally provides coverage over area A, D, and E, transmitter 102 generally provides coverage over area B, D, and E, and transmitter 104 generally provides coverage over area C, E, and F. In some cases, the coverage area of a first transmitter may be entirely enclosed within the coverage area of another transmitter, such as in building interiors and valleys. In areas where one (and only one) transmitter dominates (e.g., areas A, B, and C in FIG. 1), simulcast is effective because the other transmitters do not significantly affect receivers in those areas.
However, in "overlap" areas D, E, and F shown in FIG. 1, where the signals from two or more transmitters are approximately equal, problems can arise because destructive interference of signals occurs in these overlap areas such as areas D, E, and F. Destructive interference occurs when the two signals are equal in magnitude and 180.degree. out of phase and completely cancel each other. While there were some successes, reliable design procedures were not available.
Attempting to precisely synchronize the carrier frequencies of all simulcast transmitters does not overcome the problem because points (i.e. nodes) at which destructive summing occurred persisted for long periods of time. At such points, a mobile receiver can not receive the simulcast signal.
Deliberately offsetting the carrier frequencies of adjacent; transmitters can ensure that destructive interference does not persist at one point for an extended period of time. The slight errors in frequency displayed by high quality reference oscillators (e.g., 20 hertz errors in 100 MHz signals or a few parts in 10.sup.7) render deliberate offsetting unnecessary. Further, merely offsetting the carrier frequencies could not guarantee acceptable quality demodulation because proper alignment of the modulating signals in time is also required.
FIG. 2 displays the situation at, for example, point D in FIG. 1 when modulating waveforms are synchronized and includes coverage boundary 202 from a first transmitter and a second transmitter coverage boundary 204 from a second adjacent transmitter. An equi-signal boundary 200 exists where the signals from the first and second transmitters have approximately equal signal strengths. A more realistic equi-signal boundary would take into account natural and man-made topography and propagation conditions, and therefore would probably not be a straight line.
FIGS. 3 and 4 generally illustrate various signals as they may occur at or near the equi-signal boundary 200 as shown in FIG. 2. In particular, FIGS. 3 and 4 illustrate various aspects of modulation synchronization and how altering transmission parameters may affect the synchronization. In general, there are at least three sources which cause the signals from the first transmitter and the second transmitter to be out of synchronization: (1) timing shifts in the delivery of the modulating waveform to each of the transmitters; (2) timing shifts internal to each transmitter; and (3) timing shifts caused by propagation distances and anomalies. From the perspective of a receiver located in an overlap area, these three sources of timing shifts combine to produce an overall timing shifts between the received signals from the first and second transmitters. In current commercial practice, the summation of these three components results in time shifts of about 200 microseconds. The timing shift present in simulcast systems disadvantageously limits the baud rate at which information may be transferred. In general, FIGS. 3 and 4 will also illustrate how timing shifts prevents high baud rate transmissions.
A time line representation of a signal 306 from a first transmitter is shown in FIG. 3(A) and a signal 308 from a second transmitter is shown in FIG. 3(B), both from the perspective of a receiver located in an overlap area. Vertical dashed lines 300 represent baud intervals on the time axis. As can be seen from FIGS. 3(A) and (B), the signals 306 and 308 are frequency modulated between a high and a low frequency value and the signals 306 and 308 are exactly in phase. As will be appreciated, the timing shift between signals 306 and 308 must be small when compared to the baud interval shown in FIGS. 3(A) and (B) since signals 306 and 308 are in synchronization. Of course, as the baud interval decreases, the timing shifts will likely cause signals 306 and 308 to be out of synchronization.
FIGS. 3(C), (D), and (E) show the summation of these two signals 306 and 308 at an equi-signal boundary, such as boundary 200 in FIG. 2. FIG. 3(C) shows a composite signal 310 indicating that the frequency information remains unchanged, FIG. 3(D) shows a linear graph 312 of the relative phase difference caused by a slight carrier frequency difference between the signals from the first transmitter and the second transmitter. FIG. 3(E) shows a composite amplitude signal 314. A noise threshold is indicated by the horizontal dashed line 304 in FIG. 3(E).
Of interest, FIG. 3(E) shows the composite amplitude signal 314 dipping below the noise threshold 304 at an anti-phase condition 302 (e.g., when the relative phase angle is .+-.180.degree., as shown in FIG. 3(D)). As can be seen from FIG. 3(E), the anti-phase condition 302 caused by the slight phase shift between transmitter 1 and transmitter 2 will not cause any loss of data because the anti-phase condition persists for only a small portion of the baud interval.
The slight offset of the carrier frequencies between the first and second transmitters causes a slow drift of the relative phase of the two signals, as shown in FIG. 3(D). When the signals are .+-.180.degree. out of phase, the temporary dip in the amplitude signal may cause the loss of a few bits in the composite signal, at worst. These errors can be counteracted with a conventional error correcting code, such as is commonly known.
FIG. 4 shows a set of similar signals to those in FIG. 3, but wherein the signal 402 from the first transmitter is offset from, or out of synchronization with, the signal 404 from the second transmitter by a full baud. In particular, signal 404 lags signal 402 by one baud interval. As previously discussed, the offset of signals 402 and 404 may be caused by various timing shifts in the delivery of both signals 402 and 404 to a receiver in an overlap area. FIGS. 4(A) and (B) illustrate the extreme case where the sum of these timing shifts is equal to the baud interval shown by dashed lines 400. As can be seen in FIG. 4(C), composite signal 406 includes a period of indeterminate frequency which undesirably covers several entire baud intervals and, therefore, successful demodulation is impossible during those baud intervals. If the baud interval were increased to minimize the effect of these timing shifts, data loss would be less likely. Therefore, it can be seen that the baud rate at which good data transfer can be accomplished is limited by the timing shifts between signals delivered to receivers in overlap areas.
Through these examples, it can be seen that high degrees of modulation synchronization make it possible to obtain good data demodulation in a simulcast system. However, the baud rate limitation of simulcast systems is a significant drawback and limits system throughput.
An alternative to simulcast for wide area coverage is assignment of orthogonal, non-overlapping subdivisions of the available system capacity to adjacent areas. Subdivisions can be made in time (e.g., broadcasting the information on the same frequency in different time slots to adjacent areas), or in frequency (e.g., broadcasting the information simultaneously on different frequencies in adjacent areas). There are several problems with such orthogonal systems, however. First, orthogonal assignments require tuning the receiver to the assigned frequency or time channel for the area in which the receiver currently resides. In the broadcast services every traveler has experienced the frustration of finding the correct channel for their favorite programs. Simulcast operation avoids the need for scanning and re-tuning as the mobile unit moves between areas. Such scanning and re-tuning also disadvantageously increases mobile unit power consumption.
Second, and more serious, the orthogonal assignment approach drastically reduces the system throughput capacity as measured in bits per Hz because anywhere from 3 to 7, or possibly more, orthogonal assignments are required to obtain continuous area coverage in most conventional orthogonal systems. This waste of capacity is somewhat recouped if the same information is not needed throughout the service area because a given piece of information is sent only to those cells where it is needed.
Conventional cellular radio service is a typical example of an orthogonal system. In cellular, the same frequencies are reused in spatially separated cells to allow different data to be transmitted to different mobile units. An example of three cellular arrangements is shown in FIG. 5 where the number of cells (N) is equal to 3, 4, and 7. Each cell (i.e., A, B, C, . . . ) in conventional cellular service usually only includes a single transmitter and operates in a different frequency or time division within the communication protocol. As shown in FIG. 5, cellular service generally locates transmitters utilizing the same division (all the "A" transmitters) far enough apart to reduce the likelihood of interference between such transmitters. As the number of cells increases, the likelihood of interference decreases. For example, with N=3 as shown by arrangement 500 in FIG. 3, the distance between the coverage area of "A" cells is about 1/2 cell width, with N=4 in arrangement 502, the distance between the coverage areas of "A" cells is slightly larger, and with N=7 in arrangement 504 the distance between "A" cells is larger than the width of one cell.
However, as the number of cells increases, the length of the individual time intervals per cell decreases for time division multiplexed systems, thereby decreasing the systems total information transfer. In frequency division systems, more cells undesirably increases the frequency bandwidth required. Therefore, system throughput in bits per Hz is decreased as the number of cells increases. Furthermore, cellular systems often require an electronic "handshake" between system and mobile unit to identify the specific cell (i.e. transmitter) in which the mobile unit is located to allow capacity reuse.